An image scanner raster-scans characters and images recorded on an original and converts the scanned data into electric signals to be read.
The image scanner successively traces the gray levels of the ground color of the original, prepares a required white level according to the gray levels, and generates an analog white level signal representing the white level. The image scanner scans the original, provides analog image signals representing the scanned original, determines the gray level (between 0 and 1) of each of the analog image signals according to the analog white level signal serving as a white level of 1, and provides digital image signals representing the gray levels. The gray levels represented with the digital image signals are examined to determine whether or not the original contains recorded images as well as the concentration of the images.
The characters and images recorded on the original and read by the image scanner may be read into a computer. In this case, the gray levels of pixels read by CCD sensors of the image scanner are compared with a predetermined slice level, to binarize the read data into white and black parts. For this binarization, it is important to optimize the slice level to be compared with gray levels.